First, go up the food chain with the site you're trying to work with. If they flat-out refuse to budge, you have a couple of options:

1) Contact the airline, car rental agency, and hotel directly and ask to change your reservation. Airlines charge a fee for this ($25 - 100, depending on the carrier). I've never had a car rental agency charge me a fee for this. Unless the hotel reservation is prepaid, they should be cooperative, as well.

2) Cancel/dispute the charges and rebook somewhere else.

Good luck. We finally started using a local travel agency for our business travel because the DIY online method was eating up too much time when we needed changes.